This invention pertains generally to a system for the detection of aircraft or guided missiles, and particularly to a system using radar and infrared (IR) sensors for the detection of low flying aircraft or guided missiles in a high clutter background.
It has been proposed to combine a radar and an IR detection arrangement to provide an improved air defense system. The capabilities of the two different types of detectors then would complement each other, making it difficult, if not impossible, for attacking aircraft or guided missiles successfully to penetrate the space protected by the air defense system. It is necessary, therefore, that an IR detection arrangement be provided so that low flying aircraft or guided missiles (hereinafter sometimes referred to as "targets") not ordinarily detected by radar may be detected,even in the presence of IR clutter. IR clutter may be deemed to be IR signals from one of three different types of sources: (a) ground or the bodies of clouds; (b) scintillating objects; or (c) edges of rapidly moving clouds.
It is known in the art that IR clutter from the ground or bodies of clouds may be distinguished from targets. Thus, IR clutter from the ground or the bodies of clouds may contain frequencies that differ from the frequencies of IR signals from targets. Specifically, the frequency components in IR signals from ground clutter or the body of a cloud are usually concentrated in the lower end of the spectrum, while the frequency components in IR signals from targets are relatively high. Therefore, it is known to filter received IR signals to reject the low frequency components and to pass the high frequency components. Hopefully, then, the IR signals that are passed are deemed to be targets. Unfortunately, however, there are clutter signals that have essentially the same frequency components as a target. Thus, IR clutter from scintillating objects (i.e., objects that reflect sunlight, as, for example, glass surfaces that act as mirrors), have frequency spectra that are quite similar to the frequency spectrum of any target signal, so the known cancellation technique for ground clutter is not effective against scintillating IR clutter. Further, the frequency spectra of the edges of wind-driven clouds are quite similar to the frequency spectra of target signals, so the known cancellation technique for ground clutter is not effective against "wind-driven cloud edge" IR clutter.
It is highly desirable in a dual mode tracking system that there be a high degree of correlation between the targets as seen by a radar and the targets as seen by the IR detection arrangement. That is to say, if the complexity of the data processor in a dual mode tracking system is to be kept within reasonable bounds and if the constant false alarm rate (CFAR) of such a system is to be maintained below a reasonable level, it is necessary that IR clutter signals be eliminated.